U.S. patent application number 09/905279 was filed with the patent office on 2003-01-23 for characterization of a scan line produced from a facet of a scanning device.
Invention is credited to Carlson, Gerard J., Hirst, B. Mark, Wibbels, Mark.
Application Number | 20030016403 09/905279 |
Document ID | / |
Family ID | 25420551 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030016403 |
Kind Code |
A1 |
Carlson, Gerard J. ; et
al. |
January 23, 2003 |
Characterization of a scan line produced from a facet of a scanning
device
Abstract
A method for refining a length of a scan line, where the scan
line is produced from a facet of a scanning device. The method
comprises the steps of: (a) acquiring a plurality of scan line
lengths produced from the facet, (b) determining from the plurality
of scan line lengths, an average scan line length for the facet,
and (c) determining from the average scan line length, a scan line
length correction for the facet. A method for measuring a length of
a scan line comprises the steps of: (a) charging an electrical
current integrator to a voltage while a scan line is produced from
a facet, (b) measuring the voltage, and (c) determining from the
voltage, the length of the scan line produced from the facet.
Inventors: |
Carlson, Gerard J.; (Boise,
ID) ; Hirst, B. Mark; (Boise, ID) ; Wibbels,
Mark; (Boise, ID) |
Correspondence
Address: |
HEWLETT-PACKARD COMPANY
Intellectual Property Administration
P.O. Box 272400
Fort Collins
CO
80527-2400
US
|
Family ID: |
25420551 |
Appl. No.: |
09/905279 |
Filed: |
July 13, 2001 |
Current U.S.
Class: |
358/505 ;
358/519 |
Current CPC
Class: |
H04N 2201/04786
20130101; H04N 1/1135 20130101; H04N 2201/04798 20130101; H04N
2201/04789 20130101; H04N 1/053 20130101; H04N 2201/0471 20130101;
H04N 2201/04732 20130101; H04N 2201/04729 20130101; H04N 2201/04744
20130101 |
Class at
Publication: |
358/505 ;
358/519 |
International
Class: |
H04N 001/46; G03F
003/08 |
Claims
What is claimed is:
1. A method for refining a length of a scan line, wherein said scan
line is produced from a facet of a scanning device, comprising the
steps of: (a) acquiring a plurality of scan line lengths produced
from a facet; (b) determining from said plurality of scan line
lengths, an average scan line length for said facet; and (c)
determining from said average scan line length, a scan line length
correction for said facet.
2. The method of claim 1, wherein said facet is one of a plurality
of facets on a rotating reflector, and wherein said method further
comprises the step of obtaining a number from a cyclic counter to
identify said facet and to associate said plurality of scan line
lengths with said facet when determining said average scan line
length.
3. The method of claim 1, wherein said scanning device produces
pixels at a dot imaging frequency, and wherein said plurality of
scan line lengths is acquired from a counter that is clocked at a
rate of less than 8 times said dot imaging frequency.
4. A system for refining a length of a scan line, wherein said scan
line is produced from a facet of a scanning device, comprising: (a)
means for acquiring a plurality of scan line lengths produced from
a facet; (b) means for determining from said plurality of scan line
lengths, an average scan line length for said facet; and (c) means
for determining from said average scan line length, a scan line
length correction for said facet.
5. The system of claim 4, wherein said facet is one of a plurality
of facets on a rotating reflector, and wherein said system further
comprises a cyclic counter for providing a number for said facet to
identify said facet and to associate said plurality of scan line
lengths with said facet when determining said average scan line
length.
6. The system of claim 4, wherein said scanning device produces
pixels at a dot imaging frequency, and wherein said acquiring means
comprises a counter that is clocked at a rate of less than 8 times
said dot imaging frequency.
7. A storage medium that includes instructions for controlling a
processor to execute a method for refining a length of a scan line,
wherein said scan line is produced from a facet of a scanning
device, said storage medium comprising: (a) first instructions for
controlling said processor to acquire a plurality of scan line
lengths produced from a facet; (b) second instructions for
controlling said processor to determine from said plurality of scan
line lengths, an average scan line length for said facet; and (c)
third instructions for controlling said processor to determine from
said average scan line length, a scan line length correction for
said facet.
8. The storage medium of claim 7, wherein said facet is one of a
plurality of facets on a rotating reflector, and wherein said
storage medium comprises further instructions for controlling said
processor to obtain a number from a cyclic counter to identify said
facet and to associate said plurality of scan line lengths with
said facet when determining said average scan line length.
9. The storage medium of claim 7, wherein said scanning device
produces pixels at a dot imaging frequency, and wherein said scan
line length is acquired from a counter that is clocked at a rate of
less than 8 times said dot imaging frequency.
10. A method for measuring a length of a scan line, wherein said
scan line is produced from a facet of a scanning device, comprising
the steps of: (a) charging an electrical current integrator to a
voltage while a scan line is produced from a facet; (b) measuring
said voltage; and (c) determining from said voltage, a length of
said scan line.
11. The method of claim 10, further comprising the step of
determining from said length, a scan line length correction for
said facet.
12. The method of claim 10, wherein said facet is one of a
plurality of facets on a rotating reflector, and wherein said
method further comprises the step of obtaining a number from a
cyclic counter to identify said facet and to associate said length
of said scan line with said facet.
13. The method of claim 10, further comprising before said charging
step, the step of resetting said electrical current integrator in
response to a receipt of a reference signal that indicates a start
of said scan line produced from said facet.
14. The method of claim 13, wherein said resetting step is delayed
by a predetermined interval of time from said receipt of said
reference signal.
15. The method of claim 10, wherein said measuring step samples
said voltage in response to a receipt of a reference signal that
indicates an end of said scan line produced from said facet.
16. The method of claim 10, wherein said measuring step samples
said voltage in response to a receipt of a reference signal that
indicates a start of a scan line produced by a next facet.
17. A system for measuring a length of a scan line, wherein said
scan line is produced from a facet of a scanning device,
comprising: (a) means for charging an electrical current integrator
to a voltage while a scan line is produced from a facet; (b) means
for measuring said voltage; and (c) means for determining from said
voltage, a length of said scan line.
18. The system of claim 17, further comprising means for
determining from said length, a scan line length correction for
said facet.
19. The system of claim 17, wherein said facet is one of a
plurality of facets on a rotating reflector, and wherein said
system further comprises a cyclic counter for providing a number
for said facet to identify said facet and to associate said length
of said scan line with said facet.
20. The system of claim 17, further comprising means for resetting
said electrical current integrator in response to a receipt of a
reference signal that indicates a start of said scan line produced
from said facet.
21. The system of claim 20, wherein said resetting means performs
its respective action after a delay of a predetermined interval of
time from said receipt of said reference signal.
22. The system of claim 17, wherein said measuring means samples
said voltage in response to a receipt of a reference signal that
indicates an end of said scan line produced from said facet.
23. The system of claim 17, wherein said measuring means samples
said voltage in response to a receipt of a reference signal that
indicates a start of a scan line produced by a next facet.
24. A storage medium that includes instructions for controlling a
processor to execute a method for measuring a length of a scan
line, wherein said scan line is produced from a facet of a scanning
device, comprising the steps of: (a) first instructions for
controlling said processor to charge an electrical current
integrator to a voltage while a scan line is produced from a facet;
(b) second instructions for controlling said processor to measure
said voltage; and (c) third instructions for controlling said
processor to determine from said voltage, a length of said scan
line.
25. The storage medium of claim 24, comprising further instructions
for controlling said processor to determine from said length of
said scan line, a scan line length correction for said facet.
26. The storage medium of claim 24, wherein said facet is one of a
plurality of facets on a rotating reflector, and wherein said
storage medium comprises further instructions for controlling said
processor to obtain a number from a cyclic counter to identify said
facet and to associate said length of said scan line with said
facet.
27. The storage medium of claim 24, comprising further instructions
for controlling said processor to reset said electrical current
integrator in response to a receipt of a reference signal that
indicates a start of said scan line produced from said facet.
28. The storage medium of claim 27, wherein said instructions for
controlling said processor to reset said electrical current
integrator performs its respective action after a delay of a
predetermined interval of time from said receipt of said reference
signal.
29. The storage medium of claim 24, comprising further instructions
for controlling said processor to sample said voltage in response
to a receipt of a reference signal that indicates an end of said
scan line produced from said facet.
30. The storage medium of claim 24, comprising further instructions
for controlling said processor to sample said voltage in response
to a receipt of a reference signal that indicates a start of a scan
line produced by a next facet.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to scanning devices and, more
particularly, to a technique for characterizing a scan line that is
produced from a facet of the scanning device.
BACKGROUND OF THE INVENTION
[0002] Printing devices such as laser printers, digital
photocopiers and fax machines use a laser beam to write an image on
a photosensitive surface. The surface moves and the laser beam
scans an image by sweeping in a direction perpendicular to that of
the moving surface. This scanning action is similar to the movement
of an electron beam across a television tube or other cathode ray
tube (CRT). However, unlike a CRT, one dimension of the image, call
it the Y dimension, is controlled by the movement of the surface,
while the other dimension, call it the X dimension, is controlled
by the scanning action of the laser.
[0003] Laser imaging devices implement the scanning action by
reflecting a laser beam off a rotating reflector, often referred to
as a rotating mirror. The rotating mirror is a reflector typically
having two or more faces called facets that reflect the laser beam.
Mirrors with two, four, six or eight facets are not uncommon.
[0004] In a laser imaging system having a mirror with a plurality
of facets, the quality of an image is affected by several factors
related to the design and manufacture of the facets. An ideal
system operates within the constraints listed below.
[0005] (1) In the case of an even-sided polygonal mirror, facets on
opposite sides of the mirror are parallel to one another.
[0006] (2) The mirror has minimal run-out relative to its
rotational center.
[0007] (3) The angles formed by the facets of the polygon are
precise.
[0008] (4) The motor and bearing system run true, without
wobble.
[0009] A system that fails to meet these constraints can produce
objectionable artifacts in a printed image. These artifacts are due
to scan lines of different lengths.
[0010] In a system using a multi-facet mirror, successive facets of
the mirror produce successive scan lines of the image. Thus, a
specific facet of the rotating mirror produces specific scan lines.
For example, a four-faceted mirror will produce scan lines as shown
in Table 1, below.
1TABLE 1 Four-Faceted Mirror Facet Number Scan Line Number 1 1 2 2
3 3 4 4 1 5 2 6 3 7 4 8 1 9 2 10 3 11 4 12
[0011] Imperfections in the mirror facets can cause scan lines to
be of different lengths. For example, all the scan lines written by
a facet can be of one length, while those written by another facet
are a different length. Presently, scanners can produce scan lines
with 300 or 600 dots per inch (DPI) that vary less than one dot per
line. Nonetheless, even a minor difference in the length of a scan
line can cause periodic distortion in an image.
[0012] The beginning of each scan line is electronically
synchronized to a starting margin of an image. The synchronizing
signal is conventionally known as a "beam detect" (BD). A variation
in the scan line accumulates over the length of the scan line and
typically reaches its maximum at the end of the scan line.
[0013] An observer will usually not notice any variation in a
single line. However, a periodic pattern produced by the variation
in the scan line may interfere with a pattern of gray scale or
halftones in an image, thus creating a moir pattern. A moir pattern
typically appears as a periodic series of lines superimposed over
the image. Even though differences in the lengths of the scan lines
are less than one dot wide, the human vision system is very
sensitive to moir patterns caused by errors of less than the width
of one dot. For example, a human can detect a moir pattern caused
by a 1/4 dot error in an image produced by a 600 DPI printer. As
laser printers are called upon to print images approaching
photograph quality, gray scale and halftone patterns are used more
frequently, and the resulting images are more susceptible to
moir.
[0014] Methods exist that compensate for scanner imperfections by
lengthening or shortening the lines produced by the facets until
each line is the same length. A system applying such a method
requires knowledge of:
[0015] (1) the amount of facet-to-facet imperfection, and
[0016] (2) which facet of the mirror is reflecting the laser
beam.
[0017] Knowledge of the amount of facet-to-facet imperfection is
used to determine how much compensation is required for a
particular facet. A facet error can be characterized in terms of
the time it takes to sweep a beam across a predetermined length.
For example, a 600 DPI printer that prints a line across an
eight-inch page prints 4800 dots.
4800 dots=600 dots/inch.times.8 inches
[0018] A particular model of printer may print a dot in 50
nanoseconds (ns). Thus an eight-inch line would be printed in 240
microseconds (.mu.s).
240 .mu.s=4800 dots.times.50 ns/dot
[0019] If the nominal scan line is 240 .mu.s long, then a scanner
imperfection that causes a scan line length of 240.050 .mu.s
corresponds to a length of one extra dot. A scan line length of
239.950 corresponds to a line that is one dot shorter than the
nominal line. As humans can detect moir patterns caused by a 1/4
dot error, a facet-to-facet deviation of 12.5 ns can result in a
noticeable imaging artifact.
[0020] Knowledge of which facet of the mirror is reflecting the
laser beam is necessary so that an appropriate compensation can be
applied when a particular facet is producing a scan line. The scan
line lengths are corrected on a facet-by-facet basis so that all
the resulting printed lines on the page are the same lengths.
[0021] One technique for characterizing a scanner involves the use
of a test fixture to measure and record the scan line length of
each facet. This information is either physically written onto the
scanner, or stored into an electronic memory that is included with
the scanner. The information is subsequently recalled during a line
length correction procedure. Because the measurement is made
external to the scanner system, this technique requires additional
manufacturing steps for the characterization process, and further
requires a step for a transfer of the characterization information
from the scanner to a compensation circuit. Both steps add to the
cost of the scanner.
[0022] Another technique for characterizing a scanner involves the
use of a high-speed counter to accurately measure the scan line
length. Dots are printed at a rate based on a dot frequency. For
example, if a dot is printed in 50 ns, its effective dot frequency
is {fraction (1/50)} ns or 20 MHz. A counter that measures a line
in a scanning system is typically clocked at least eight times
faster than the dot frequency in order to provide 1/8 dot
resolution. In this example, the clock frequency would be at least
160 MHz.
160 MHz=8.times.20 MHz
[0023] A higher clock frequency would provide an even more accurate
measurement of the scanner error. For example, a 1 GHz clock would
provide a timing resolution of 1 ns, which corresponds to {fraction
(1/50)} of a dot. Disadvantageously, the higher the clock speed at
which a circuit operates, the higher is the cost of electronic
circuitry and the greater is the potential for radiated noise.
[0024] An existing technique for identifying which facet of a
mirror is reflecting the laser beam is to tag one facet so that the
facet can be detected by a sensor. For example, the tag might be a
physical mark that is optically sensed. Assume that facet #1 is
tagged. The sensor will detect facet #1, and thereafter a beam
detect circuit counts subsequent facets until the mirror makes a
full rotation bringing facet #1 into printing position again.
Disadvantageously, this technique requires a means for tagging a
facet, a sensor for detecting the tag, and wiring to communicate
the facet information to the compensation circuitry.
[0025] Accordingly, there is a need for a method of characterizing
a scanner facet error that is performed within a scanner system
without requiring high-speed electronic circuitry.
[0026] There is also a need for a method of identifying which facet
is reflecting a laser beam without requiring a sensor for detecting
a particular facet.
SUMMARY OF THE INVENTION
[0027] In a first embodiment of the present invention a method is
provided for refining a length of a scan line, where the scan line
is produced from a facet of a scanning device. The method comprises
the steps of: (a) acquiring a plurality of scan line lengths
produced from the facet, (b) determining from the plurality of scan
line lengths, an average scan line length for the facet, and (c)
determining from the average scan line length, a scan line length
correction for the facet.
[0028] In a second embodiment of the present invention a method is
provided for measuring a length of a scan line, where the scan line
is produced from a facet of a scanning device. The method comprises
the steps of: (a) charging an electrical current integrator to a
voltage while a scan line is produced from a facet, (b) measuring
the voltage, and (c) determining from the voltage, the length of
the scan line produced from the facet.
[0029] Systems are also provided to perform each of the
methods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a flowchart of a method for refining a length of a
scan line in accordance with the present invention;
[0031] FIG. 2 is a schematic diagram of a system for refining a
length of a scan line in accordance with the present invention;
[0032] FIG. 3 is a flowchart of a method for measuring a length of
a scan line in accordance with the present invention;
[0033] FIG. 4 is a schematic diagram of a system for measuring a
length of a scan line in accordance with the present invention;
[0034] FIG. 5 is a timing diagram showing how delaying the
resetting of an integrator effects the integration interval of the
integrator;
[0035] FIG. 6 is a block diagram of a simplified laser scanning
system; and
[0036] FIG. 7 is a diagram of an imperfect four-sided
reflector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] FIG. 1 is a flowchart of a method 100 for refining a length
of a scan line in accordance with the present invention. The scan
line is produced from a facet of a scanning device. Typically, the
facet is one of a plurality of facets on a rotating polygon mirror.
As described below, method 100 includes the steps of: (a) acquiring
a plurality of scan line lengths produced from a facet, (b)
determining from the plurality of scan line lengths, an average
length of the scan line for the facet; and (c) determining from the
average scan line length, a scan line length correction for the
facet. The method begins with step 110.
[0038] In step 110, the method obtains a number from a cyclic
counter, such as a modulo counter. The modulo of the counter is the
same as the number of facets on the mirror. The output of the
counter is used to identify the facet that is producing the scan
line and to associate a length of a scan line with a particular
facet when determining an average scan line length as described in
step 120, below. However, the counter is not intended to absolutely
identify any particular facet, but instead, an arbitrary
relationship is initially established between the output of the
counter and each of the facets, and the relationship is maintained
for the duration of the method.
[0039] For example, assume that a mirror has four facets, i.e., A,
B, C and D. Accordingly, the counter will cycle through the values
0, 1, 2 and 3. At the commencement of this method, the facets and
the counter will assume some arbitrary relationship, such as that
shown in Table 2, below. In this example, a counter output of 2
will identify facet A.
2TABLE 2 Arbitrary Relationship Between Facets and Modulo Count
Actual Facet Output at Counter A 2 B 3 C 0 D 1
[0040] The relationship between the facets and the counter, once
established, remains fixed during the method described herein. The
counter is synchronized to advance as the mirror rotates to each
subsequent facet. In a preferred implementation, the
synchronization can be provided by a reference signal such as a
beam detect, as mentioned earlier. The method then advances to step
115.
[0041] In step 115, the method acquires a length of a scan line
produced by the facet. As the mirror rotates, the method acquires a
plurality of scan line lengths produced from the facet. At this
step, any conventional technique for measuring a scan line length
can be employed, however, in the preferred embodiment, the scan
line length is acquired from a counter that is clocked at a rate of
less than 8 times the dot imaging frequency. The method then
advances to step 120.
[0042] In step 120, the method determines from the plurality of
scan line lengths acquired in step 115, an average scan line length
for the facets. The method then advances to step 125.
[0043] In step 125, the method determines from the average scan
line length determined in step 120, a scan line length correction
for the facet. That is, the method determines a compensation that
will be applied to either lengthen or shorten the scan line
produced by the facet.
[0044] In the preferred embodiment, when applying the compensation
to a scan line, the modulo counter used to identify a facet in step
110, is also used to identify the facet during correction of the
scan line. That is, the output of the modulo counter is used to
identify a particular facet and an appropriate compensation is
applied when that facet is producing a scan line.
[0045] The method illustrated in FIG. 1 is executed for each of the
facets on the mirror. The method can be executed in a single pass
in which data is collected for all of the facets, and the
respective average scan lengths are then determined, or the method
can be executed as a loop.
[0046] The correction for a scan line is achieved by shifting the
time at which a pixel is printed, thus resulting in a shift of the
point at which the pixel is printed so that adjacent raster lines
are aligned with one another. For example, a pixel at row 5, column
4000 will properly align with a pixel at row 6, column 4000.
[0047] FIGS. 6 and 7, in association with their related
descriptions, present a technique for determining a compensation
value. However, the present invention is not limited to the use of
any particular technique of line length compensation, but instead,
any suitable compensation technique can be employed.
[0048] FIG. 6 shows a simplified laser scanner system. A laser beam
605 reflects off of a rotating mirror 610 and impinges on a surface
of a photoconductor 615. Rotating mirror 610, shown here as
rotating in a clockwise direction, causes laser beam 605 to sweep
across the surface of photoconductor 615 in a left to right
direction.
[0049] Prior to impinging on photoconductor 615, the reflected
laser beam 605 activates a beam detect sensor 620 located to the
left of a left margin 625. When laser beam 605 crosses left margin
625, a laser control system (not shown) begins to modulate laser
beam 605 with print image data to print a line. The modulation
continues until laser beam 605 reaches a right margin 630.
[0050] Rotating mirror 610 has four surfaces, namely A, B, C, and
D. Due to slight irregularities in the surfaces of A, B, C and D,
the printed lines will have different lengths. The four different
line lengths La, Lb, Lc, and Ld correspond to the mirror face from
which they were produced. In practice, correcting lenses (not
shown) are placed between rotating mirror 610 and photoconductor
615.
[0051] FIG. 7 shows a four-sided mirror 705, which is imperfect
because its four sides are not of equal length. In this figure, the
imperfections are grossly exaggerated for the sake of clarity.
Because the lengths are not equal, the angles .THETA.a, .THETA.b,
.THETA.c, and .THETA.d, traced out by the faces A, B, C, and D are
different from one another.
[0052] Mirror 705 rotates at a constant angular velocity, .omega..
The amount of time that each face spends reflecting the laser beam
is equal to its corresponding angle .THETA.a, .THETA.b, .THETA.c,
or .THETA.d, divided by .omega..
[0053] Table 3 shows the time that each facet spends reflecting the
laser beam.
3 TABLE 3 Facet Angular velocity Facet Angle Time per facet A
.omega. .THETA..sup.a .THETA..sup.a/.omega. = Ta B .omega.
.THETA..sup.b .THETA..sup.b/.omega. = Tb C .omega. .THETA..sup.c
.THETA..sup.c/.omega. = Tc D .omega. .THETA..sup.d
.THETA..sup.d/.omega. = Td
[0054] A wide facet, such as facet A in FIG. 7, sweeps through a
large angle .THETA.a, and thus reflects the laser beam for a long
period of time .THETA.a/.omega.=Ta. In contrast a narrow facet,
such as Facet B, sweeps through a smaller angle .THETA.b, and
reflects the laser beam for a shorter period of time
.THETA.b/.omega.=Tb. As a result, the time between the beam detect
pulse when facet A first rotates into the laser, and the second
beam detect when facet B rotates into the laser will be longer than
the time between beam detects for facets B and C.
[0055] The present invention, by accurately measuring these beam
detect periods, can determine which facets are wider or narrower A
compensation method can then adjust the laser dot frequency to
compensate for the facet variation. An exemplary compensation
method is described below.
[0056] A fraction of the time that a facet is reflecting the laser
beam is used for writing printable data between the left and right
margins. Call this fraction c. A given facet such as facet A, will
write data for cTa, while facet B will write for cTb. For a
writeable area between margins L, the laser beam writing velocity
in this writeable area is:
Sx=L/(cTx)
[0057] where x refers to the facet A, B, C or D. Sx has the units
of distance per time such as inches per second.
[0058] A dot density D, is measured in dots per unit distance, for
example, 600 dots per inch. Multiplying D by Sx yields units of
dots per time.
[0059] Typical laser control circuitry uses one cycle of a digital
clock to write a single dot. The digital clock, also called a video
clock, can then be calculated from the following formula:
F=D*L/(c*Tx)
[0060] As D, c and L are design constants of the scanning device,
the beam detect period, Tx, measured by the invention, can be used
to calculate a frequency Fx, needed for line length compensation. x
refers to the facet A, B, C or D
[0061] Stated qualitatively, a wider facet will write the writeable
area for a longer period of time, and the writing rate or frequency
of the laser must be slower. Conversely, a narrower facet will
sweep the laser beam quickly through the writeable area and the
laser must write more quickly using a higher frequency.
[0062] The following example demonstrates a compensation
method.
[0063] Given:
[0064] L=8.000 inches
[0065] D=600 dots/inch
[0066] c=0.5
[0067] Ta=502(10.sup.-6) seconds measured by the invention.
[0068] Tb=499(10.sup.-6) seconds measured by the invention.
[0069] Tc=502(10.sup.-6) seconds measured by the invention.
[0070] Td=499(10.sup.-6) seconds measured by the invention.
[0071] The video frequency required for each facet is:
[0072] Fa=600*8/(0.5*502(10.sup.-6))=19.123506 MHz
[0073] Fb=600*8/(0.5*499(10.sup.-6))=19.238477 MHz
[0074] Fc=600*8/(0.5*502(10.sup.-6))=19.123506 MHz
[0075] Fd=600*8/(0.5*499(10.sup.-6))=19.238477 MHz
[0076] Any convenient frequency synthesis technique can be used to
control the video frequency on a facet by facet basis.
[0077] FIG. 2 is a schematic diagram of a system 200 for refining a
length of a scan line in accordance with the present invention. The
scan line is produced from a facet of a scanning device. Typically,
the facet is one of a plurality of facets on a rotating polygon
mirror. As described below, system 200 includes components for (a)
acquiring a plurality of scan line lengths produced from a facet,
(b) determining from the plurality of scan line lengths, an average
scan line length for the facet, and (c) determining from the
average scan line length, a scan line length correction for the
facet.
[0078] System 200 includes a clock 205, a scan length counter 210,
a facet modulo counter 215, a latch 225, a microprocessor 230,
memory 235 and a scan line length compensation circuit 245.
[0079] System 200 receives a reference signal that indicates a
particular point in a scan line produced from a facet. In the
embodiment shown in FIG. 2, the reference signal is a beam detect
(BD) 220, which indicates the start of a scan line. Beam detect 220
is routed to scan length counter 210, facet modulo counter 215,
latch 225 and microprocessor 230.
[0080] Clock 205 is a clock circuit. It provides a clock to scan
length counter 210.
[0081] Scan length counter 210 has a CLOCK input driven by clock
205, and a CLEAR input driven by beam detect 220. It acquires and
measures a length of a scan line produced from a facet. Scan length
counter 210 is cleared by beam detect 220, and thereafter counts up
to indicate the length of the scan line. In the preferred
embodiment, it is clocked at a rate of less than 8 times the dot
imaging frequency of the scanner, but it can operate at a higher
frequency. Because system 200 determines an average of a plurality
of scan line lengths from a facet, system 200 can resolve a scan
line error to a fraction of a pixel notwithstanding the use of a
relatively slow clock frequency from clock 205. The output of scan
length counter 210 is routed to latch 225.
[0082] Latch 225 receives a scan length count from scan length
counter 210, and it has a LOAD input driven by beam detect 220.
Beam detect 220 causes latch 225 to latch the scan length count
from scan length counter 210. The latched scan length count is
routed to microprocessor 230.
[0083] Facet modulo counter 215 has a COUNT input driven by beam
detect 220. It is a cyclic counter for providing a number for the
facet to identify the facet and to associate the scan line length
with the facet when determining the average scan line length. Its
modulo is the same as the number of facets on the mirror. Facet
modulo counter 215 is incremented upon each occurrence of beam
detect 220. The counter is not intended to absolutely identify any
particular facet, but instead, it establishes an arbitrary count
that is subsequently used to identify each of the facets. The
output of facet modulo counter is routed to microprocessor 230.
[0084] Microprocessor 230 receives scan length count data from
latch 225, and the facet number from facet modulo counter 215. It
also has an INTERRUPT input that is driven by beam detect 220.
Microprocessor 230 operates in association with memory 235, which
stores data and instructions for execution by microprocessor 230.
Upon the occurrence of beam detect 220, microprocessor 230 is
interrupted. It loads scan length count data from latch 225, and
the facet number from facet modulo counter 215. Thereafter it
determines an average of a plurality of scan line lengths for the
facet. The average scan line length for the facet is routed from
microprocessor 230 to compensation circuit 245.
[0085] Compensation circuit 245 receives the average scan line
length from microprocessor 230. It uses the average scan line
length to determine a scan line length correction for the
facet.
[0086] System 200 performs the operations described above for each
of the facets of the mirror. The scan line length data for each of
the facets is acquired by scan length counter 210 and latch 225,
and then stored into memory 235 for subsequent use by
microprocessor 230.
[0087] In the preferred embodiment, when applying the compensation
to a scan line facet modulo counter 215 is also used to identify
the facet during correction of the scan line. That is, the output
of facet modulo counter 215 is used to identify a particular facet
and an appropriate compensation is applied when that facet is
producing a scan line.
[0088] System 200 can be implemented with discrete components,
firmware, or a single processor such as a digital signal processor.
It can also be implemented in software and executed on a
general-purpose computer. Furthermore, while the procedures
required to execute the invention hereof are indicated as already
loaded into a memory, such as memory 235, they may be configured on
a storage medium, such as data memory 240, for subsequent loading
into memory 235.
[0089] FIG. 3 is a flowchart of a method 300 for measuring a length
of a scan line, where the scan line is produced from a facet of a
scanning device. Typically, the facet is one of a plurality of
facets on a rotating polygon mirror. As described below, method 300
includes the steps of: (a) charging an electrical current
integrator to a voltage while a scan line is produced from a facet,
(b) measuring the voltage, and (c) determining from the voltage, a
length of a scan line produced from the facet. The method begins
with step 310.
[0090] In step 310, the method performs the step of charging an
electrical current integrator to a voltage while a scan line is
produced from a facet. A current integrator charges to a voltage as
a function of time.
v(t)=1/C.intg.i(t)dt
[0091] Where: v(t)=voltage at time (t)
[0092] C=capacitance of integrator
[0093] i(t)=current at time (t)
[0094] Accordingly, the voltage (v) to which the integrator charges
is a function of the time (t), i.e. an integration interval, during
which the scan line is produced. As described below, the scan line
length can be determined from the voltage (v). The method then
advances to step 315.
[0095] In step 315, the method performs the step of measuring the
voltage (v) to which the integrator charged. The method then
advances to step 320.
[0096] In step 320, the method performs the step of determining
from the voltage, a length of a scan line produced from the facet.
That is, it determines the time (t) required for the integrator to
charge to the voltage (v), and relates the time (t) to the scan
line length. The scan line length can thereafter be used to
determine a scan line length correction for the facet.
[0097] Preferably, prior to step 310, the method performs the step
of resetting the electrical current integrator in response to a
receipt of a reference signal, such as a beam detect, that
indicates a start of the scan line produced from the facet. The
beginning of the period of integration then coincides with the
beginning of the scan line.
[0098] Method 300 can achieve better resolution by delaying the
resetting step for a predetermined interval of time from the
receipt of the reference signal. For example, if the integration
interval is centered around an expected time of occurrence of a
second beam detect signal, then the integration interval will be
shorter. If the voltage (v) in step 320 is measured using an A/D,
as described below in association with FIG. 4, then by delaying the
integration interval, an A/D of fewer bits can be used and a
greater resolution per bit is also provided. That is, the smaller
the integration interval, e.g., 256 ns rather than 250 s, then the
fewer the number of bits required of the A/D, and the greater the
resolution per bit. The relationship between the reference signal
and the resetting step are described in greater detail in
association with FIG. 5, below.
[0099] Since the voltage (v) measured during step 315 is a function
of the time at which the measurement is taken, consideration must
be given to the timing of the measurement. The method can sample
the voltage (v) in response to a signal that indicates an end of
the scan line produced by the facet, or in response to a receipt of
a signal that indicates a start of a scan line produced by a next
facet. In either case, the signal provides a convenient point of
reference that is related to the timing of the production of a scan
line.
[0100] In practice, method 300 is executed for each of a plurality
of facets on the mirror. In such a case, method 300 performs the
additional step of obtaining a number from a cyclic counter to
identify the facet and to associate the scan line length with the
facet when determining the scan line length.
[0101] FIG. 4 is a schematic diagram of a system 400 for measuring
a length of a scan line in accordance with the present invention.
The scan line is produced from a facet of a scanning device, and
typically, the facet is one of a plurality of facets on a rotating
polygon mirror. As described below, system 400 includes components
for: (a) charging an electrical current integrator to a voltage
while a scan line is produced from a facet, (b) measuring the
voltage, and (c) determining from the voltage, a length of a scan
line for the facet.
[0102] System 400 includes an electrical current integrator 410, a
sample/reset logic circuit 415, a facet modulo counter 420, a
sample and hold circuit 425, an analog to digital converter (A/D)
427, a microprocessor 430, a memory 435 and a compensation circuit
440.
[0103] System 400 receives a reference signal that indicates a
particular point in a scan line produced from a facet. In the
embodiment shown in FIG. 4, the reference signal is a beam detect
(BD) 405, which indicates the start of a scan line. Beam detect 405
is routed to sample/reset logic circuit 415, microprocessor 430 and
facet modulo counter 420.
[0104] Integrator 410 has a RESET input that is driven by a reset
control signal from sample/reset logic circuit 415. After being
reset, it charges to a voltage (v) while a scan line is produced
from a facet. The length of a scan line can be determined from the
voltage (v). The voltage (v) out of integrator 410 is routed to
sample and hold circuit 425.
[0105] Sample/reset logic circuit 415 has an input to receive beam
detect 405, and it provides a reset control signal to integrator
410 and a sample control signal to sample and hold 425.
Sample/reset logic circuit 415 issues the reset control signal to
integrator 410 a predetermined interval of time after receiving
beam detect 405. It issues the sample control signal to sample and
hold circuit 425 at a time that indicates a convenient point in the
production of a scan line. For example, the sample control signal
can be triggered by beam detect 405, which may indicate either the
end of the scan line that was produced from the facet or a start of
a scan line produced by the next facet on the mirror. The timing
relationships between beam detect 405, the reset control signal and
the sample control signal are described below in association with
FIG. 5.
[0106] Facet modulo counter 420 has a COUNT input driven by beam
detect 405. It is a cyclic counter for providing a number for the
facet to identify the facet and to associate the scan line length
with the facet when determining the scan line length. Its modulo is
the same as the number of facets on the mirror. Facet modulo
counter 420 is incremented upon each occurrence of beam detect 405.
The counter is not intended to absolutely identify any particular
facet, but instead, it establishes an arbitrary count that is
subsequently used to identify each of the facets. The output of
facet modulo counter 420 is routed to microprocessor 430.
[0107] Sample and hold circuit 425 has an input to receive voltage
(v) from integrator 410, and an input for the sample control signal
from sample/reset logic circuit 415. Sample and hold circuit 425
samples and holds voltage (v), and provides it as an input to A/D
427.
[0108] A/D 427 receives voltage (v), which is an analog signal,
from sample and hold circuit 425. It performs an A/D conversion of
voltage (v) and provides a digitized voltage as an input to
microprocessor 430.
[0109] Microprocessor 430 has data inputs to receive the digitized
voltage from A/D 427, and the count from facet modulo counter 420.
It also has an INTERRUPT input that is driven by beam detect 405.
Microprocessor 430 operates in association with memory 435, which
stores data and instructions for execution by microprocessor 430.
Upon the occurrence of beam detect 405, microprocessor 430 is
interrupted. It reads the digitized voltage data from A/D 427, and
the facet number from facet modulo counter 420. Thereafter, it
determines the scan line length of the facet. The scan line length
is routed from an output of microprocessor 430 to an input of
compensation circuit 440.
[0110] Compensation circuit 440 has an input to receive the scan
line length data from microprocessor 430. From the scan line length
data, it determines a scan line length correction for the
facet.
[0111] In the preferred embodiment, when applying the compensation
to a scan line facet modulo counter 420 is also used to identify
the facet during correction of the scan line. That is, the output
of facet modulo counter 420 is used to identify a particular facet
and an appropriate compensation is applied when that facet is
producing a scan line.
[0112] System 400 performs the operations described above for each
of the facets of the mirror. The scan line length data for each of
the facets is measured for each of the facets and then stored into
memory 435 for subsequent use by microprocessor 430.
[0113] System 400 can be implemented with discrete components. It
can also be implemented so that the functions of some components,
for example sample/reset logic circuit 415, facet modulo counter
420, A/D 427, microprocessor 430, memory 435 and compensation
circuit 440 are performed by firmware, or a single processor such
as a digital signal processor. These functions can also be
implemented in software and executed on a general-purpose computer.
Furthermore, while the procedures required to execute the invention
hereof are indicated as already loaded into a memory such as memory
435, they may be configured on a storage medium, such as data
memory 445, for subsequent loading into memory 435.
[0114] FIG. 5 is a timing diagram showing how delaying the
resetting of an integrator effects the integration interval of the
integrator. Assume that an active-low beam detect (/BD) occurs at
the beginning of a scan line from a facet of a rotating mirror that
has four facets as shown by the progression of the Current Facet.
The integrator is reset at the time of a Reset pulse and sampled at
the time of a Sample pulse.
[0115] Two timing examples are presented below. In these examples,
a beam detect pulse 505 indicates the start of a scan line from
Facet 1, and a beam detect pulse 506 indicates the start of a scan
line from Facet 2.
[0116] In the first example, a Reset pulse 510 occurs shortly after
the leading edge of beam detect pulse 505, a Sample pulse 515
occurs shortly after the leading edge of beam detect pulse 506, and
shortly thereafter, a Reset pulse 511 resets the integrator voltage
to 0 volts in preparation of acquiring a subsequent sample. The
integrator charges to a voltage during an Integration Interval 520
that spans the period of time from Reset pulse 510 to Sample pulse
515. The length of Integration Interval 520 can be determined from
the voltage, and in turn, the scan line length of Facet 1 can be
determined from the length of Integration Interval 520.
[0117] In a second example, a delayed Reset (Reset.sub.D) pulse 525
occurs a predetermined time after beam detect pulse 505. As in the
first example, Sample pulse 515 occurs shortly after the leading
edge of beam detect pulse 506. The integrator charges to a voltage
during a delayed Integration Interval (Integration Interval.sub.D)
530 that spans the period of time from Reset.sub.D pulse 525 to
Sample pulse 515. The length of Integration Interval.sub.D 530 can
be determined from the voltage, and in turn, the scan line length
of Facet 1 can be determined from the sum of the predetermined
period of delay and the length of Integration Interval.sub.D
530.
[0118] The voltage output of an integrator is a function of the
duration of an integration interval. Integration Interval 520 is
longer than Integration Interval.sub.D 530. Accordingly, the
voltage sampled at the end of Integration Interval 520 will be
greater than the voltage sampled at the end of Integration
Interval.sub.D 530. If Integration Interval 520 was 240 us, then a
16-bit D/A is required in order to obtain a resolution of 3.67
ns.
3.67 ns=240 s/2.sup.16
[0119] However, if Reset.sub.D pulse 525 occurs after a delay of
239 us, then Integration Interval.sub.D 530 is 1 us and a less
expensive A/D of 10 bits can resolve the interval down to 1 ns.
1 ns=1 us/2.sup.10
[0120] That is, the use of a delay in this case allows a use of a
less expensive A/D.
[0121] The resolution of a measurement of a scan line length can be
further improved as described above in association with FIGS. 3 and
4. By delaying the resetting of the integrator, the integration
interval will be shortened, an A/D of fewer bits can be used and a
greater resolution per bit is also provided.
[0122] It should be understood that the foregoing description is
only illustrative of the invention. Various alternatives and
modifications can be devised by those skilled in the art without
departing from the invention. Accordingly, the present invention is
intended to embrace all such alternatives, modifications and
variances that fall within the scope of the appended claims.
* * * * *